Human spines are formed from vertebrae which are separated and cushioned from each other by discs. The discs consist of a fibrous outer envelope containing a gel-like fluid. The discs are subject to large forces, which may vary from about 175 pounds when a person is at rest to as high as about 500 pounds. For example, a person who lifts a 15 pound weight one foot in front of such person, using a bending movement, can generate nearly 500 pounds of force on his or her spine. Because of the high forces on them, spinal discs commonly rupture, particularly as they deteriorate with age.
Various attempts have been made to deal with the problem of a ruptured disc. One standard procedure is to remove the disc and fuse the vertebrae which were formerly separated by the disc. A difficulty with this is that relative motion between the two vertebrae is no longer possible, causing both stiffness in the spine and difficulties in areas above and below the fused discs.
Other attempts to deal with the problem have involved removing the ruptured disc and replacing it with an artificial resilient pad made e.g. of high density polyethylene or of a silicone material. Such pads tend to wear out rapidly, since a spinal disc typically undergoes goes between one and five million cycles of compression and extension per year. In addition they provide no compliance or natural restoring force, and they do not offer the constraints to movement which are provided by a natural disc.
Another attempted solution has been made to use ball bearings in place of discs. However no satisfactory method has been developed for retaining the ball bearings in place.